The invention generally relates to three-terminal switching power circuits performing functions of thyristors and triacs.
Thyristor, also known as SCR (silicon controlled rectifier), is a three-terminal unidirectional switching power device. The cathode is the reference terminal. The thyristor can be triggered by a low-level positive voltage applied to its gate if a positive voltage is applied to its anode. The thyristor is automatically turned off by reducing the anode voltage or reducing the principal current below a hold current.
Similarly, triac is a bidirectional switching power device. The triac can be triggered by a low-level voltage applied to its gate and having the same polarity as the voltage applied to its anode. It can be turned off under the same conditions as the thyristor.
A number of devices relating to thyristor and triac and similar in operation thereto is also available, e.g. PUT, SBS and UJT. A GTO (gate turn-off thyristor) is of interest as it can be turned off by an adequate gate voltage.
The thyristors and triacs are extremely slow. They also overwhelmingly require a high gate current. Furthermore, the forward voltage drop is relatively high due to the basic four layer structure. Triacs require the gate voltage having the same polarity as the anode voltage. The thyristors and triacs once triggered cannot be turned off by the gate control. The GTOs can be turned off by applying a gate voltage of an opposite polarity within a specific range and require a substantial reverse gate-drive current.